1. Field of Invention
This invention relates to a cartridge-type fuse used in an electric circuit such as those commonly found in an automobile or the like. In particular, the invention relates to a cartridge-type fuse having improved fusing time-current characteristics when an overcurrent such as a short circuit current flows through the fuse.
2. Description of Related Art
Accompanying FIG. 3 shows a conventional cartridge-type fuse assembly 1 commonly used to protect an electric circuit in an automobile or the like. As shown in FIG. 3, the fuse assembly 1 has a pair of terminals 3a, 3b connected by a fuse element 5 and resides in a housing 7 made from insulated heat-resisting resin. The fuse assembly 1 also includes a transparent cover 11 which covers the opening 9 at the upper portion of the housing 7. The housing 7 has terminal receiving chambers 4a, 4b, for receiving terminals 3a, 3b, formed inside the housing below a fuse element space 6 for containing the fuse element 5. When the terminals 3a, 3b are positioned within the terminal receiving chambers, the fuse element 5 is seated within the fuse element space 6 and straddles the terminal receiving chambers 4a, 4b. The fuse element 5 includes a fusible (current responsive) portion which fuses open when an overcurrent or short-circuit passes through it, thereby, opening the circuit connected to the fuse to protect the circuit. The condition of the fuse element including the fusible portion can be readily observed through the transparent cover 11.
Generally, in a conventional fuse as described above, the fusing time, or the time elapsing from the melting of the fusible portion of the fuse element 5 to the final opening of the circuit, is correlated to the current flow as shown in FIG. 4. Namely, the fusible portion in the conventional fuse is fused open in a very short period of time when subjected to a current flow rate in excess of 200 percent of the fuse capacity rating. However, this fusing time is relatively long when the conventional fuse is subjected to a current flow below 200 percent of the fuse capacity rating since the conventional fuse resists fusing at these lower current flows. The fusing time is also extended when an intermittent overcurrent such as a short circuit current, as opposed to a continuous current, flows through the fusible portion resulting in repeated heating and radiating of the fuse. Furthermore, in the case of an intermittent short circuit current flow, electric wires which have protective insulating coating will not radiate heat as readily as the fusible portion of the fuse element, thereby, generating smoke in the worst case.